From 285b6aad1aee686b6677817cef71fa31af3bb073 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mamy=20Andr=C3=A9-Ratsimbazafy?= <mamy_github@numforge.co>
Date: Sun, 16 Feb 2020 22:18:22 +0100
Subject: [PATCH] Add cmov according to Milagro / hash_to_curve spec

---
 constantine/math/bigints_raw.nim | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/constantine/math/bigints_raw.nim b/constantine/math/bigints_raw.nim
index 1adaff8..6d4e2b3 100644
--- a/constantine/math/bigints_raw.nim
+++ b/constantine/math/bigints_raw.nim
@@ -217,6 +217,16 @@ func setZero*(a: BigIntViewMut) =
   ## It's bit size is unchanged
   zeroMem(a[0].unsafeAddr, a.numLimbs() * sizeof(Word))
 
+
+func cmov*(a: BigIntViewMut, b: BigIntViewAny, ctl: CTBool[Word]) =
+  ## Constant-time conditional copy
+  ## If ctl is true: b is copied into a
+  ## if ctl is false: b is not copied and a is untouched
+  ## Time and memory accesses are the same whether a copy occurs or not
+  checkMatchingBitlengths(a, b)
+  for i in 0 ..< a.numLimbs():
+    a[i] = ctl.mux(b[i], a[i])
+
 # The arithmetic primitives all accept a control input that indicates
 # if it is a placebo operation. It stills performs the
 # same memory accesses to be side-channel attack resistant.